The planet of Nibiru is the home-world of the Anunnaki race in ancient Mesopotamian religions.
According to these Mesopotamian legends, Nibiru is originally from a solar system near the star Sirius. Somehow, the planet was ejected from that solar system and caught by ours. As it's new orbit placed it very far from the sun, the Anunnaki needed a plan to retain heat for the planet. They decided to do this by lining the upper atmosphere with gold dust. To do this, they needed a large supply of gold. An expedition lead to Earth found that it was of high gold content, so they began mining for it.
Crash with TiamatEdit
At the time before Nibiru entered our solar system, the legends maintain that our planet Earth (called Tiamat to the ancient Babylonians) was almost twice as large and had a different orbit than today. The planet Nibiru came in on its highly unusual orbit, and a moon orbiting it impacted in the pacific ocean, apparently destroying an entire continent. Although we have never found evidence of that continent's existence, some believe this to be how the Moon was created, because scientists are certain the Moon was formed by the Earth. The resulting displacement of crust around that area of impact (and some in other parts of the planet, no doubt) made earth much smaller than it is today. The knowledge of this event apparently came to the ancient peoples from their Anunnakian gods.
Rob Solarion's Lecture in Texas